Intermediate valent Tm compounds: A superfluid in a condensed excitonic state

Exciton condensation in the form of an excitonic insulator has been postulated by Mott already in 1961. The experimental realization of such a state in intermediate valent TmSe_1-xTe_x compounds, however, had to wait until 1990 [J. Neuenschwander, P. Wachter, Phys. Rev. B 41, 12693 (1990)]. Now it appears essential that an intermediate valent state is a prerequisite for the exciton condensation because a narrow hybridized 4f band is necessary. The new and exciting facts are thermodynamic measurements on TmSe_0.45Te_0.55 single crystals at low temperatures and high pressure. At ambient conditions this material is a narrow gap semiconductor which can be transformed under high pressure (14 kbar) into an intermediate valent metal. Below 250 K and between 5 and 13 kbar the excitonic state is formed. In this case, the thermal conductivity shows a behavior which has only been observed so far in the superfluid phase of He II.